ARM: socfpga: fix build error due to secondary_startup

After commit 02b4e275 (ARM: v7 setup function should invalidate L1
cache) the soc specific secondary_startup is removed, causing build
failures:

../arch/arm/mach-socfpga/platsmp.c: In function 'socfpga_a10_boot_secondary':
../arch/arm/mach-socfpga/platsmp.c:66:140: error: 'socfpga_secondary_startup' undeclared (first use in this function)
../arch/arm/mach-socfpga/platsmp.c:66:140: note: each undeclared identifier is reported only once for each function it appears in

To fix, use the generic secondary_startup.

Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>